previewer: pdfjs plugin addition

* Adds pdfjs previewer.

Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>
previewer: custom invenio viewer

* Customize the pdfjs viewer for invenio.

Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>